This is some of the worst kind of gatekeeping.
(1) Physics as a discipline needs people who are curious, engaged, open-minded, and comfortable delving into the unknown.
(2) Anyone can study physics for any reason.
(3) Anyone can pursue an interest in anything at any age.
I hate to say it, but as a former physics major, if you actually want to go into physics you should be the kind of person who finds this level of material completely obvious at 9 years old.
It's absolutely worth studying -- but physics only needs people who are math freaks.